Molecular-level understanding of phosphorus transformation with long-term phosphorus addition and depletion in an alkaline soil

With the increasing pressure on phosphorus (P) resources and global water eutrophication, the development of sustainable land management is pushed forward by detailed knowledge on the nature and fate of phosphorus (P) in agricultural soils under various long-term fertilizer regimes. Using bulk- (solution P-31 nuclear magnetic resonance; P K-edge X-ray absorption near-edge, XANES) and micro- (X-ray fluorescence and XANES) spectroscopic techniques, we investigated the molecular speciation of P in an alkaline soil from plots under wheat-maize rotation with or without chemical P fertilizers for 26 years, since 1990. The long-term experimental results show that P buildup and drawdown were mainly in the inorganic forms in response to P addition and depletion. The added P increased P accumulation in the forms of brushite and deoxyribonucleic acid that preferentially depleted without P addition, indicating that these species were phytoavailable. Although P forms as hydroxyapatite and orthophosphate monoesters were increased under P fertilization, no significant depletions of these P forms were observed when P fertilization was withheld. In contrast, iron-associated P increased with and without P fertilization. These results from this long-term experimental site facilitate understanding of dynamics and availability of fertilizer P and the existing legacy P in soils, with important implications for sustainable P management in these soils and similar soils in other regions.